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Need help with 4.3L V6 Volvo in runabout

I took my 2004 Regal 1800LSR in for skeg repair about 3 weeks ago. The shop was to: (1) weld on new skeg (2) replace original impeller (3) replace a leaking water circulation pump gasket. Also, the original impeller was completely in-tact when removed. I ran the boat the day after I got it back and it overheated for the 1st time ever. I immediately took it back to the shop. He said I must have sucked up some gunk, so he cleaned it out and gave it back to me the next day. Again, I ran the boat and it overheated.

Fed up with Shop "A", I decided to take it to Shop “B” b/c I no longer trust the 1st place. Shop "B" has since replaced the thermostat - didn’t fix it. They pulled the lower end off and inspected the hoses near the intake to ensure they’re connected correctly – didn’t fix anything. They pulled the impeller shop “A” installed and put in a “more robust” impeller – didn’t fix it.

Today, they will take it back out on the water and run it to get it up to temp and then use a laser temp gun to measure the actual temp of the engine because they’re questioning if the temperature sending unit is bad. They’ve admitted they’re baffled by this overheating condition.

Symptoms: cools OK while idling and at WOT, but nowhere in between. For example, it overheats at planning speed.

Please help me guys! What the heck is wrong with this motor? It was not overheating when I originally took it to Shop “A”.

just a stupid thought,did you take the thermostat out and try it? I've got new ones bad out of the box.if he took the circulation pump off,did he damage it putting it back on?just throwing some things out there. let us know what you find, and good luck!

actually the second shop pointed out the mounting bolts on the water circ pump still have the factory paint covering them, so I think shop A tried to pull one over on me. Still no word from the shop - just waiting to get it back still.

Update:
Shop put a new thermostat in and that didn't resolve the overheating issue either.

Next, the tech used a laser temp gun to measure the temperature at varrious points of the engine when the gauge and horn indicated it was overheating. When the gauge was reading 225 degrees and the horn was blowing, he was only getting temperature readings of around 170 all around the engine. He didn't find 1 spot where it read hot, yet the gauge indicated an overheating condition. So, he now believes it's a matter of a faulty temp sending sensor. He said there's 3 on the engine and he's going to replace them 1 by 1 until the problem is fixed. This way, he's not replacing the "good" sensors and potentially opening a new can of worms in the process.

First, it is entirely possible that one of your temp gauge senders went bad. But for it to have gone bad at the exact time you had your impeller changed seems like more coincidence than I would be willing to accept. I am wondering if mechanic "A" disturbed something.

The second issue is that a temp sender is most likely to be electric. If it failed, I would expect it to always show bad. But you are describing it acting normal whenever you're idling and at wide open throttle and showing a false overheat when the engine is in the middle RPM range. If that is really how it's working, that would be an odd way for a failing temp sender to work. Maybe it was going junk and its operation was unrelated to RPM. That would make more sense.

I do think mechanic "B" is going about this in a reasonable and logical way using the non-contact thermometer. I'll bet you get to the bottom of this if you stick with his methods.

A couple of different people have now told me that Volvo motors MUST have Volvo brand impellers. I told this to 'mechanic B' a couple of weeks ago, but we just wrote it off as a wives tale. Now that so many other things have been ruled out, I brought this question up again to the mechanic. So far, it's had a Cierra impeller installed by 'mechanic A' and then a Johnson Pump installed by 'mechanic B'. On Saturday, I drove to Volvo parts dealer and bought the impeller myself and then delivered it to my mechanic's shop. The mechanic called me and confirmed he received it and then proceeded to tell me there actually IS a visual difference between the Volvo impeller and the other 2 we've already tried - the Volvo impeller is slightly taller! He's installing it today and will report back to me ASAP.